1. Field of the Invention
This invention relates to a subtraction processing method used when reproducing radiation images. This invention particularly relates to a so-called temporal (time difference) subtraction processing method used when reproducing radiation images wherein the image of a specific structure or part of an object is extracted from the image of the whole object by subtracting the image signal of a radiation image of the whole object obtained without injection of contrast media from the image signal of a radiation image in which the image of the specific structure of the object is enhanced by the injection of contrast media.
2. Description of the Prior Art
Carrying out subtraction processing on radiation images has heretofore been known. When performing subtraction processing, two radiation images recorded under different conditions are photoelectrically read out and digital image signals are obtained which represent the radiation images. The parts of the image signals which represent corresponding picture elements in the radiation images are then subtracted from each other, thereby to obtain a difference signal representing an image of a specific structure or part of the object represented by the radiation images. With the subtraction processing method a difference signal is obtained by subtracting two digital image signals from each other, and from the difference signal, the radiation image of a specific structure can be reproduced.
As one of the subtraction processing methods, the so-called temporal (time difference) subtraction processing method has heretofore been known. In the temporal subtraction processing method, the image of a specific structure in an object is extracted from the image of the whole object by subtracting the image signal representing a radiation image (a masked image) of an object obtained without injection of contrast media from the image signal representing a radiation image (a live image) in which the image of the specific structure in the object is enhanced by the injection of contrast media.
The image signals used and on which subtraction processing is carried out may be obtained by photoelectrically reading out radiation images recorded on, for example, X-ray photographic film. On the other hand, as disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318 and 4,387,428 and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use a stimulable phosphor sheet in radiation image recording and reproducing systems. With the proposed radiation image recording and reproducing systems, an image signal representing a radiation image can be obtained directly, and processing, such as the developing of film, is not required. Therefore, it is advantageous for the radiation images on which subtraction processing is to be carried out, i.e. a masked image and a live image, to be recorded and read out by using one of the above proposed radiation image recording and reproducing systems.
Specifically, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored during exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
In the aforesaid radiation image recording and reproducing systems, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation, which has passed through an object such as the human body, in order to store a radiation image of the object thereon, and is then scanned with stimulating rays which cause it to emit light in proportion to the amount of energy stored during exposure to radiation. The light emitted by the stimulable phosphor sheet upon stimulation thereof is photoelectrically detected and converted into an electric image signal.
In general, when radiation images (a masked image and a live image), each stored on a stimulable phosphor sheet, are read out in the course of carrying out a temporal subtraction processing method, it is required that read-out conditions such as read-out gains and scale factors for both the masked image and the live image be adjusted so that they are equal to each other. Otherwise, part of the image of an object which does not belong to the specific structure of an object to be extracted remains unremoved when an image is reproduced from the difference signal obtained by carrying out the subtraction processing method.
However, it is often desired to view the masked image and the live image themselves. In such cases, it is desired that the read-out conditions for the masked image and those for the live image be adjusted to values appropriate for each image, which values may be different from each other. This requirement is incompatible with the requirement to make the read-out conditions for the image signals equal when carrying out subtraction processing.
The same problems arise also when image signals are obtained by reading out radiation images recorded on X-ray photographic film.